In a tactical environment, RF communications may be protected from jamming and signal interception by periodically changing transmission frequency (either by frequency agility, direct sequence spread spectrum, or both) and encrypting transmitted data with a time dependent encryption algorithm. In order for two nodes to communicate with each other, they must be accurately synchronized in time.
In an ad hoc tactical network, the location and activation of radio network nodes may occur in a non-predictable and random manner. When the network employs direct sequence spread spectrum (DSSS) operation and time based encryption algorithms, it is essential that the nodes have a mechanism for discovering each other to synchronize their internal clocks. Furthermore, if a GPS knowledgeable node is present, the aggregate non-GPS based time is slowly pulled toward GPS based time in order to facilitate multi-tier operation. For the purpose of discovery, nodes that have no known neighbors (e.g. isolated nodes) may listen for a late net entry (LNE) signal from other nodes. After receiving the LNE signal, the nodes exchange time information and align each other with the same network time (e.g. become associated nodes). These nodes may also synchronize their network time with GPS based time, if available.
Because these nodes operate in a changing tactical environment, however, the members of an associated group of nodes may periodically lose and regain connectivity with each other. In the presence of a GPS knowledgeable node, which is causing the entire net of nodes to slowly gravitate toward GPS based time, unconnected nodes may be left in a state which cannot synchronize with other net members.
Wireless nodes employ clock synchronization techniques when operating in a wireless personal area network (WPAN) and a wireless local area network (WLAN). Bluetooth devices, which support low bandwidth and short distance communications, are examples of WPAN devices. For timing purposes, Bluetooth devices communicate and exchange data, by using master/slave clock synchronization methods which allow synchronization between node neighbors that are one-hop away from each other.
IEEE 802.11 WLAN standard specifies approaches for time synchronization of infrastructure-based networks and independent networks. For the infrastructure-based networks, IEEE 802.11 provides master/slave clock synchronization that uses a fixed node, the access point (AP), as a master. In an independent network, a mobile node transmits a beacon message with a chosen beacon period. Each receiving node updates its clock with the value in the received beacon message, if the received value is greater than its current local time. If the received value is less than the current local time, the received value is discarded.
One type of radio, known as the Near Term Data Radio (NTDR), manufactured by ITT, is an example of a tactical ad-hoc radio employing TOD synchronization. To assist synchronization with a common TOD, the radio uses three TOD message types (Cold Start, LNE and In-Net). In the Cold Start (CS) mode, the radio within the network uses a fixed TRANSEC. This permits the radio within the network to listen to CS messages without an initial TOD reference. Upon receiving a CS TOD update message, the radio extracts the transmitted time and uses it to update its own TOD. The radio then enters the LNE mode and selects a LNE TRANSEC, which is within 6 minutes from the In-Net TOD. The radio remains in LNE mode until it is within 20 msec of the transmitted time, after which it transitions into the In-Net mode. An In-Net message is then used for normal message transmission.
TOD synchronization of the NTDR is modified in another type of radio, known as the Small Unit Operation (SUO) radio, also manufactured by ITT. The basic functionality of CS, LNE and In-Net modes of the NTDR are replaced by functions of isolated, in-sync and associated modes employed in the SUO radio. One feature of the SUO radio is its TOD synchronization which allows a roaming node with local GPS to immediately synchronize with the current (non-GPS based) net time, and then slowly pull the net time toward the GPS based time.
A description of TOD synchronization follows. An isolated node devotes its resources to discovering and synchronizing time with its RF neighbor(s). To do this, the isolated node issues LNE signals at a high rate and periodically monitors the LNE channel for messages from other nodes. LNE signals are transmitted on the highest frequency channel available to the radio. The LNE signal uses a fixed code division multiple access (CDMA) spreading code and a fixed TRANSEC from the current key fill.
An associated node issues LNE signals at a low rate. Since all associated nodes issue LNE messages independently, an isolated node quickly detects a LNE signal, although associated nodes are issuing LNE messages at a low rate. After detecting an LNE message, the isolated node synchronizes with the associated nodes.
Messages, known as Packet Radio Organizational Packets (PROP), are used to discover new RF neighbors and exchange information for automatic transmission power (ATP) calculations. After a node completes TOD synchronization, it begins issuing PROP messages to identify other RF neighbors for establishing bi-directional communications.
FIGS. 1 and 2 illustrate TOD message transmission waveforms. FIG. 1 shows a TOD synchronization waveform that is transmitted by a node when it has isolated status. FIG. 2 shows a TOD synchronization waveform that is transmitted by the node when it has associated status.
An isolated node attempts to discover neighbors by listening for both the neighbor's LNE and RTS (request-to-send) transmissions. Since these two signals do not use the same spreading codes and frequencies, the transceiver is typically assigned to listen for one of these signals and the auxiliary receiver listens for the other. In a battery powered system, however, the use of two receiving devices is wasteful of power. In order to conserve power, the auxiliary receiver in an isolated node is placed into standby and the transceiver alternately switches between the frequencies and codes used for LNE and RTS. Since the transmitting station sends both an LNE and an RTS in an even epoch, followed later by the same transmission in an odd epoch, the single receiver approach detects the new RF neighbor, even if the epoch counters of the two nodes are out of phase with each other. This continues until the node becomes associated with its RF neighbors. During isolated status, PROP message (S1) and LNE message (S2), as shown in FIG. 1 are selected randomly in order to minimize the possibility of other nodes being able to intercept a local node's emissions, and in order to avoid collision.
The NTDR system includes two TOD modes, namely a GPS based mode and a brigade-time-head (BTH) based mode. In the BTH based mode, the network TOD is anchored to a master clock (BTH time). If the BTH starts the network prior to arrival of a node having GPS based time, the network operates solely as a BTH network. If a node acquires GPS signals prior to the BHT starting the network, the network operates as a GPS based network. No solution exists to move a BTH network, or non-GPS based network, toward a GPS based network, after non-GPS based TOD time and GPS based time differ by more than one epoch (100 msec).
Conventional TOD synchronization in the SUO system is superior to that of the NTDR system, when operating in a volatile environment characterized by rapidly appearing/disappearing RF neighbors. Conventional TOD synchronization, nevertheless, leads to a fragmented communications network, in which one group of nodes may be synchronized to one TOD and another group of nodes may be synchronized to another TOD.